1. Field of the Invention
The present invention relates in general to petroleum wells, and in particular to a petroleum well having a casing which is used as a conductive path to transmit wireless spread spectrum communications between surface equipment and a downhole module used to measure physical characteristics of a petroleum formation or condition of well structures.
2. Description of Related Art
Several methods have been devised to place electronics, sensors, or controllable valves downhole along an oil production tubing string, but all such known devices typically use an internal or external cable along the tubing string to provide power and communications downhole. It is, of course, highly undesirable and in practice difficult to use a cable along the tubing string either integral to the tubing string or spaced in the annulus between the tubing string and the casing. The use of a cable presents difficulties for well operators while assembling and inserting the tubing string into a borehole. Additionally, the cable is subjected to corrosion and heavy wear due to movement of the tubing string within the borehole. An example of a downhole communication system using a cable is shown in PCT/EP97/01621.
U.S. Pat. No. 4,839,644 describes a method and system for wireless two-way communications in a cased borehole having a tubing string. However, this system describes a communication scheme for coupling electromagnetic energy in a TEM mode using the annulus between the casing and the tubing. This inductive coupling requires a substantially nonconductive fluid such as crude oil in the annulus between the casing and the tubing. Therefore, the invention described in U.S. Pat. No. 4,839,644 has not been widely adopted as a practical scheme for downhole two-way communication. Another system for downhole communication using mud pulse telemetry is described in U.S. Pat. Nos. 4,648,471 and 5,887,657.Although mud pulse telemetry can be successful at low data rates, it is of limited usefulness where high data rates are required or where it is undesirable to have complex, mud pulse telemetry equipment downhole. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 4,468,665; 4,578,675; 4,739,325; 5,130,706; 5,467,083; 5,493,288; 5,576,703; 5,574,374; and 5,883,516. Similarly, several permanent downhole sensors and control systems have been described in U.S. Pat. Nos. 4,972,704; 5,001,675; 5,134,285; 5,278,758; 5,662,165; 5,730,219; 5,934,371; and 5,941,307.
Due to the limited success of wireless communication within a borehole, the current use of downhole measurement and control equipment is minimal. A lack of downhole measurement and control restricts the ability to maximize economic return by optimizing production of the well.
It would, therefore, be a significant advance in the operation of petroleum wells if an alternate means for providing communications within a well were provided. More specifically, it would be advantageous if downhole physical characteristics of the formation could be easily communicated to the surface of the well. This information could then be used to increase the aggregate recovery of formation reserves, and would thereby optimize production of the well.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein, it is incorporated by reference for background purposes and indicative of the knowledge of one of ordinary skill in the art.